1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly, to a semiconductor memory for use as a magnetic random access memory utilizing the tunneling magnetoresistive (TMR) effect which uses a toggle write system.
2. Description of the Related Art
In order to practically use a magnetic random access memory, it is necessary to restrict a TMR effect between MTJ (magnetic tunnel junction) elements and to eliminate disturbance during a write operation.
As one technique for achieving this, a toggle write system has been noticed (refer to U.S. Pat. No. 6,545,906, for example). In this write system, there is used an MTJ element, wherein a free layer is composed of two ferromagnetic layers which are weakly exchange-coupled with each other. In addition, an axial direction for facilitating magnetization of the MTJ elements is set in a direction which is different from a direction in which two write lines orthogonal to each other extend.
A write operation is always carried out by feeding a write current in a predetermined direction to the two write lines whatever a data value may be.
In the toggle write system, a data value changes every time one write operation is carried out for an MTJ element targeted for a write operation.
Therefore, in this system, before carrying out a write operation, it is necessary to read data on an MTJ element targeted for a write operation and grasp a state of that MTJ element.
Namely, in the case where write data is identical to data in the MTJ element targeted for a write operation, there is no need for carrying out a write operation. Only when the present data differ from the write data, only one write operation is carried out.
In a read operation, in general, a read electric potential obtained by an MTJ element targeted for a read operation is compared with a reference electric potential, and a data value is determined.
This reference electric potential is generated from a reference cell (MTJ element) for storing data “1” and a reference cell (MTJ element) for storing data “0”.
For example, these two reference cells are connected in parallel, and an intermediate electric potential between a read electric potential obtained by the cell for storing data “1” and a read electric potential obtained by the cell for storing data “0” are defined as a reference electric potential.
However, in a magnetic random access memory using a toggle write system, in the case of generating the reference cell for storing data “1” and the reference cell for storing data “0”, it is necessary to discuss a method for setting a data value in response to each of these reference cells.
This is because the state of the MTJ element of the magnetic random access memory using the toggle write system depends on the state of the MTJ element before a write operation, and the state of the MTJ element cannot be determined only by a write operation, whereas, for example, in die sort testing before shipment, the states of the reference cells are different depending on cells.
Moreover, it is impossible to control the magnetization direction of the MTJ element in the way of fabrication because of an exchange coupling.
Therefore, in setting the data value in response to the reference cell, for example, all the reference cells connected to one of a pair of reference cell bit lines must be reliably set to “1”, and all the reference cells connected to the other must be reliably set to “0”.
However, if a large amount of time and troubles is required to provide these settings, it causes high manufacturing cost and is inconvenient.
Hence, with respect to a semiconductor memory in which the state of the memory cell (for example, MTJ element) cannot be determined by carrying out a write operation as long as a read operation is not carried out in advance, if there exists a sequence for automatically providing setting of a data value in response to a reference cell, the existence of the sequence is very effective in reduction of the manufacturing cost.